This invention relates generally to an electrophotographic printing machine, and more particularly concerns a color electrophotographic printing machine for reproducing color transparencies.
In the process of electrophotographic printing, a photoconductive member is charged and exposed to a light image of an original document. The light image irradiates the charged portion of the photoconductive surface selectively discharging the charge thereon. In this manner, an electrostatic latent image is recorded on the photoconductive surface corresponding to the informational areas at the original document. A development system positions a developer mix of carrier granules and toner particles in contact with the electrostatic latent image. The toner particles are attracted electrostatically from the carrier granules to the latent image forming a toner powder image on the photoconductive surface. Thereafter, the toner powder image is transferred from the photoconductive surface to a sheet of support material. After the toner powder image has been transferred to the sheet of support material, a fuser permanently affixes the toner powder image thereto.
Numerous types of machines have been developed to mechanize the reproduction of microfilm. U.S. Pat. No. 3,424,525 issued to Towers et al. in 1969; U.S. Pat. No. 3,542,468 issued to Blow, Jr. in 1970; and U.S. Pat. No. 3,547,533 issued to Stokes et al. in 1970 all describe machines which produce an enlarged hard copy from a microfilm input.
With the advent of color electrophotographic printing, it became highly desirable to reproduce color transparencies or microfilm as pictorial quality opaque or hard copies. Multicolor electrophotographic printing is essentially the same as the process heretofore described. However, in multicolor electrophotographic printing, each cycle is for a discrete color contained within the original document. Hence, successive light images are filtered recording different color electrostatic latent images on the photoconductive surface. Each electrostatic latent image corresponds to a single color contained within the original document. The single color electrostatic latent images are developed with toner particles of a color complementary to the color of the filtered light image. These toner particles are transferred from the latent image to a sheet of support material in superimposed registration with one another. The multi-layered toner powder image on the sheet of support material is subsequently permanently affixed thereto forming a color copy of the original document.
As previously indicated, it is highly desirable to have the capability of reproducing color transparencies or microfilm. This may be achieved by projecting a light image of the color transparency through a screen disposed on the machine platen. The modulated light image is filtered as it passes through the machine optics to selectively discharge the charged portion of the photoconductive surface. Successive single color electrostatic latent images are formed and developed. These powder images are transferred to the sheet of support material, in superimposed registration with one another, resulting in the copy corresponding to the light image of the color transparency being projected through the machine optics. A Fresnel lens and screen a-c is positioned on the platen. The Fresnel lens converges the diverging light rays of the light image, while the screen modulates the light image. Machines of the foregoing type are more fully described in copending application Ser. No. 540,617 filed Jan. 13, 1975.
It has been found that, in general, a color electrophotographic printing machine does not produce satisfactory color copies when used in conjunction with a slide projector containing a tungsten lamp. The problem lies in the red separation and is attributable to the relatively broad red separation filter employed in the electrophotographic printing machine. For example, the 50% transmittance point of the red fiter may be at 575 namometers. The energy between 575 namometers and 610 namometers causes color containing magenta to be excessively dark. This produces a copy having excessive cyan in colors containing magenta. However, it is desirable to employ such a filter in order to satisfactorily reproduce colored opaque originals. Thus, there are two conflicting objectives, i. e. reproduction of opaque copies which require a broad red separation filter, and the reproduction of color transparencies which require a narrower red separation filter.
Accordingly, it is a primary object of the present invention to improve the copy quality of color transparencies being reproduced in an electrophotographic printing machine without degrading the quality of opaque copies being reproduced.